This invention relates generally to methods for forming video discs, and, more particularly, to methods for producing video disc masters and stampers used in forming video disc replicas.
Optically-readable video disc replicas are useful in storing vast quantities of information, usually in the form of a frequency-modulated (fm) video signal, with a high recording density. The fm signal is typically stored in a surface of the replica as a sequence of spaced pits or bumps arranged in a succession or substantially circular and concentric recording tracks. Each pit and adjacent space between pits represent one cycle of the fm signal.
A recording master typically includes a glass substrate having a disc-shaped, planar surface, with a thin photoresist recording layer overlaying it. Information is recorded in the recording layer by focusing an intensity-modulated writing beam of light onto the layer using a radially-movable objective lens, as the master is rotated at a prescribed rate. The intensity of the beam is modulated in accordance with the fm signal such that it is alternately greater than and less than a predetermined threshold at which photoresist is exposed.
Developing the exposed photoresist recording layer produces a sequence of spaced pits arranged in a succession of circular and concentric recording tracks. In one known previous developing technique, a number of different solutions were dispensed onto the layer as it was rotated at a relatively low velocity of about 75 to 100 rpm. First water was dispensed to pre-wet the layer, then both water and developer solution were dispensed for a duration of about 5 to 10 seconds to partially develop the layer, and finally developer solution alone was dispensed for a duration of about 20 seconds to fully develop the layer. The developed photoresist layer was rinsed with water to eliminate residual developer solution, and the rinsed layer was then dried by rotating it at a relatively high velocity of about 750 to 1000 rpm. The photoresist layer was preferably derived from Shipley AZ 1350 photoresist, and the developer solution was an inorganic alkali such as potassium hydroxide, sodium hydroxide, or trisodium phosphate. The developer solution was preferably AZ developer solution, which is believed to be principally trisodium phosphate, having a normality of about 0.230 to 0.240.
Disc-shaped stampers, for use in injection molding video disc replicas, are derived from the developed recording masters. In one known prior technique for making such stampers, a first metallic film having a uniform thickness of about 500 to 600 .ANG. was vapor deposited onto the developed recording layer, in a chamber pressure of about 1.times.10.sup.-5 torr. A second metallic film was thereafter electroplated onto the first film, and the two films together formed an integral metallic layer that could be separated from the underlying master recording. Residual photoresist material was removed from its undersurface using a suitable solvent, to form the stamper. Both films were preferably formed of nickel.
Although the methods described above have proven generally satisfactory as a part of an overall method of making video disc replicas, these replicas sometimes provide playback signal-to-noise ratios that are lower than believed possible. The microscopic structures of these replicas are believed to differ somewhat from an ideal sequence of sharply defined pits. There is thus a need for improved methods for making video discs having microscopic structures that conform better to an ideal sequence of sharply defined pits and thereby provide improved playback signal-to-noise ratios. The present invention fulfills this need.